The Virginia General Assembly established Preston County on January 19, 1818, taking the land from Monongalia County. The new county was named for James Patton Preston, then governor of Virginia. Already, the Indians had retreated, forts Morris and Butler were no longer in use, the American Revolution and the War of 1812 were past, and the settlers could devote themselves to improving the quality of their lives.

Preston County lies mainly within the Appalachian Plateau, with elevations varying from 873 feet to 3,216 feet. The Cheat River divides the county into two sections, the eastern being a little larger. Preston County borders Pennsylvania in the north and Maryland in the east, and the West Virginia counties of Monongalia, Taylor, Barbour, and Tucker in the west and south. The Mason-Dixon Line is its northern boundary. The county consists of 651.4 square miles in eight magisterial districts: Grant, Kingwood, Lyon, Pleasant, Portland, Reno, Union, and Valley. The population in 1818 was about 3,000, and in 2020, it was 34,216, the most in its history.

Kingwood, named for the majestic trees growing in the area at the time, is the county seat of Preston County. A stone courthouse replaced the log buildings previously used. One of the courthouses that replaced the 1820s building was burned by an arsonist in 1869, destroying most of the county records for the years 1818-69.

In early days, the northern part of the county was served by the National Road (present U.S. 40), which ran through Pennsylvania just above Preston's border, linking the area to Cumberland, Baltimore, and Wheeling. Indian trails and packhorse routes were supplemented with the building of the Northwestern Turnpike (present U.S. 50 in southern Preston County), to connect Winchester and the Ohio River. Over these roads, teamsters carried away agricultural products and brought back commercial goods.

In the 1850s, the coming of the Baltimore & Ohio Railroad provided the means to ship timber products, such as staves, and coal from Preston's vast resources. New towns grew up along the east-west railroad. The industrial age brought danger as well as prosperity. At Newburg, a thriving coal and railroad town, the explosion at Orrel Coal Company's shaft in January 1886 killed 39 men.

The earliest school was at German Settlement (now Aurora). Subscription schools preceded public or "free" schools. The first two academies for higher education were at Brandonville and Kingwood. In 1850, the two academies had three teachers and 70 pupils. The curriculum seems to have varied, possibly according to the specialties of available teachers. Terms were for about five months each. Courses included languages, surveying and the sciences, grammar, geography, and arithmetic with varying rates of tuition. Later, each magisterial district had a high school, except Pleasant District; Portland had two. By 2000, some of these high schools had been torn down, and those that survive became middle schools. One new high school was built for all county students, Preston High at Kingwood.

Following the presidential election of 1860, there was great agitation as Virginia considered whether to join the South in seceding from the Union. Preston citizens gathered at Kingwood and agreed to stand by the Union. Their representatives to the secession convention in Richmond were William G. Brown and James McGrew. Both remained strong for the Union before and after the vote in which the Virginia majority decided to secede. Brown and McGrew escaped Richmond with their lives, returning to Preston to rally the people against secession. They and others were among those who organized the Union sympathizers into a new state. Many Preston men gave their lives in the Civil War, mostly on the side of the Union. Preston Countians in large numbers have answered the calls for service in World Wars I and II, the Korean War, Vietnam, Desert Storm, Iraq, and other duty posts.

The county has progressed in many areas. A hundred years after the coming of the railroad, construction of Interstate 68, crossing the northern part of the county, eased travel between Monongalia County and Maryland. Construction of roads and bridges has been expensive and challenging due to the rugged terrain. While agriculture, coal, and timber are still important parts of the economy, so are education, health care, technology, tourism, and historic preservation. The Communications Satellite Corporation's earth tracking station at Etam provides a link in a communications system of international importance.

In 2023, Preston County's largest employers were the county school system, Vandalia Health (CAMC), Walmart, C. W. Wright Construction, and the state National Guard.

Preston County is known throughout West Virginia for its robust winters. Sometimes cold weather strays into the other seasons as well. The great frost of June 5, 1859, killed crops in the fields. They were replanted with hardy buckwheat, which was successful. For decades buckwheat was a staple crop, and buckwheat pancakes have remained a Preston county favorite since that time. Thousands gather for several days every September in Kingwood to celebrate the Buckwheat Festival, with coronations, parades, and other events while they consume prodigious quantities of pancakes, local sausage, and maple syrup. This is also the occasion for some of the largest of many annual family reunions that bring people back to their Preston County homes.